"Section 30 orders are made by the Queen, on the advice of the UK Government, having first been approved in draft by the Scottish Parliament, House of Commons and House of Lords".

But in order to do so, she needs the UK Government to agree to a Section 30 order, which gives the Scottish Parliament the right to hold a legally-binding vote.

The rallying call to union supporters comes as SNP deputy leader told delegates a second referendum on Scottish independence is going to happen.

"In other words, the margin will be more than 45-55, which is why I am not entirely sure that this referendum will go ahead at any point".

For the SNP this argument about who has the right to decide when or if Scotland can have another referendum is an example of why Scotland should leave the UK.

She is launching a "Plan for Britain". "We've had a great history together, I believe we have a great future together", she said.

In her speech at the Conservative Spring Forum in Cardiff on Friday, she said that the SNP was passing the buck between Brussels, London and Edinburgh. So, will she talk about holding a vote in the future?

Support for Scottish independence is at its highest ever but it might not be the best time for Scottish First Minister Nicola Sturgeon to hold a new referendum, a survey by ScotCen's Scottish Social Attitudes said on Wednesday (15 March).

"Right now we should be working together, not pulling apart", May told British television. "So I'm up for continued discussions".

" The 2014 referendum was legal and fair, but a wildcat version - as so many SNP members seem to want - would be anything but". That would have no legal standing and it could be challenged in the courts.

But doing so would run the risk of a landslide win for independence, potentially handing the SNP the political initiative.

Mundell said there would be no negotiations on Section 30, a procedure that allows the Scottish parliament to legislate for an independence referendum.

He declined to clarify if or when a vote could take place.

"If the Bill was then passed there would be a four-week "standstill" period prior to Royal Assent, in which any of the Law Officers - the Lord Advocate in the Scottish Government, and the Advocate General and Attorney General in the UK Government - could ask the Supreme Court to decide whether the Bill was within competence".

Deputy First Minister John Swinney will address domestic Scottish matters as the parties in Scotland prepare for May's local government elections.